Photovoltaic devices convert light into usable electrical energy due to what is known as the photovoltaic effect. Light absorbed by an active region of semi-conductor material generates electrons and holes which are separated by a built-in electric field in the photovoltaic device resulting in the generation of an electric current known as the photocurrent and a voltage known as the photovoltage. The generated electrons flow toward the region of semi-conductor material having N-type conductivity while the holes flow toward the region of semi-conductor material having P-type conductivity or a metal layer.
Basic types of hydrogenated amorphous silicon solar cells incorporating an intrinsic or undoped region are disclosed in U.S. Pat. No. 4,064,521, issued Dec. 20, 1977 to Carlson, the disclosure of which is incorporated herein by reference. The undoped hydrogenated amorphous silicon disclosed in the aforementioned U.S. Pat. No. 4,064,521 patent is slightly N-type when fabricated at an optimum substrate temperature from about 250.degree. C. to about 350.degree. C. The slightly N-type nature implies a donor-like defect in the intrinsic hydrogenated amorphous silicon region. The space charge region of the solar cell incorporating the aforementioned intrinsic layer could be increased if the region were not slightly N-type. Increasing the width of the space charge layer by incorporating a P-type dopant into the intrinsic amorphous silicon region is disclosed in U.S. Pat. No. 4,217,148, issued Aug. 12, 1980 to Carlson, the disclosure of which is incorporated herein by reference.
Carlson has attacked this problem by doping the intrinsic layer with diborane(B.sub.2 H.sub.6), wherein the diborane is present in the glow discharge atmosphere in an amount between about 0.00001 percent to about 0.003 atom percent of the glow discharge atmosphere. Carlson teaches that this doping level of diborane increases the width of the space charge layer, thereby reducing the space charge density under illumination to substantially neutral. However, a problem encountered with diborane doping in larger amounts is the reduction in the band gap as compared to intrinsic or undoped amorphous hydrogenated silicon. In addition, diborane doped material has inferior transport properties and a high density of defects, thus the combination of these two effects results in a significant loss of photogenerated carriers due to recombination in the P-layer for cells illuminated through the P-layer. Both of these effects can be ameliorated by using material doped with phosphine and illuminating through the N-layer. However, in order to use the N-layer as a window, it is necessary to modify the intrinsic layer without degrading its transport or absorption properties. The Carlson patent does not teach this important concept.